In most internal combustion engines, the valves that control cylinder ports for intake and exhaust are actuated using cams mounted on a cam shaft. Rocker arm assemblies are configured to convert the rotational motion of the cams into linear motion through which the valves open and close. The cams may be shaped in view of the timing with which it is desired to have the valves open and close.
The rocker arm assemblies form force transmission pathways between the cams and the valves. Valve lash is a gap or clearance that occurs within one of those pathways over the course of cam shaft rotation. There may be an optimal or preferred amount of lash. Too little lash may result in valve leakage or damage to moving parts. Too much lash may result in improper timing, noise, or excessive wear.
A variety of factors may affect lash. Among those factors are manufacturing tolerances, thermal expansion, and wear. In view of those factors, most engines include means for adjusting valve lash. In some engines, the lash adjustment means is designed for manual lash adjustment to be performed after assembly and again later during maintenance. Other engines use hydraulic lash adjusters that adjust lash automatically and dynamically while the engines are operating.